Dendritic cells are specialized antigen-presenting cells that are unique in their capacity to efficiently sensitize naive T lymphocytes to antigen. DCs can also affect the quality of T cell responses. For instance, DCs that secrete high levels of IL-12 can functionally polarize CD4+ T cells into IFN-gamma-high IL-4/IL-5-low TH1 type effectors. IL-12 can also enhance the capacity of sensitized CD8+ T cells to directly recognize and kill tumor targets through a mechanism that includes dramatic enhancement of T cell functional avidity (i.e. antigen sensitivity). This finding places a new emphasis on IL-12 secretion by DCs as a strategy to improve anticancer vaccines. IL-12 secretion by mature DCs is conditional, however, requiring 2 signals to achieve maximized IL-12 production. The first signal is a priming signal and is supplied by IFN-gamma. The second is a maturation stimulus supplied by LPS or CD40 ligand. We have recently shown that in vitro transcribed ssRNA mimicking structures associated with bacterial RNA can substitute for IFN-gamma as a priming signal to maximize IL-12 secretion by human monocyte-derived DCs. This finding presents the unique opportunity to formulate 2nd generation RNA vaccines that not only deliver antigenic information to DCs but also modulate their function so as to maximize direct tumor recognition and killing properties of DC-sensitized T cells. It also offers the chance to characterize a completely novel discriminatory faculty of the innate immune system. This proposal therefore seeks to understand and exploit bacterial RNA structures that induce IL-12 secretion in DCs for the directed goal of improving anti-cancer vaccines. The specific aims are 1) To define the structural parameters of ssRNA necessary to prime DC for IL-12 secretion and to incorporate them into a MART-1-encoding RNA-based vaccine; 2) To define the signal transduction pathways activated by ssRNA that regulate IL-12 secretion by DCs, and 3) To evaluate the adjuvant properties of ssRNA for modulating DC and T cell function. It is expected that these studies will shed light on a previously uncharacterized mode of discrimination by the innate immune system that will have practical applications in the formulation of RNA-based vaccines and vaccine adjuvants for cancer and infectious diseases.